Within the packaging industry, there is a progressive change towards the use of containers of plastic material. This relates to both containers for beverages, including carbonated beverages, and containers for foods. As far as foods are concerned, there is an express desire in the art also to be able to employ containers of plastic material for the storage of preserved foods. In all of these fields of application, the insufficient barrier properties of the plastic material--and in particular its insufficient capacity to prevent the passage of gases, for example oxygen, vaporized liquids such as water vapor etc. entail that the shelf-life and durability of the products stored in the containers will be far too short.
A number of proposals have been put forward in the art to solve the above problem, but, hitherto, the proposed technique has failed to meet established demands of cost in combination with barrier properties in order that containers of plastic material may successfully be employed within the above-outlined sectors. Examples of solutions proposed in the art are laminates in which two or more layers of plastic material are combined with one another and in which the material in each layer possesses properties which entail that, for instance, gas penetration, light penetration or moisture penetration are reduced. Solutions in which, for example, a metal such as aluminum is encapsulated between the plastic materials or, for instance, forms the inner surface of the container have also been suggested in the art. Such a solution is expensive and makes it difficult, if not impossible, to apply molding techniques conventionally employed in the plastic industry. Solutions in which barrier material other than metal is applied interiorly or in layers between the plastic material have further been proposed. Such solutions suffer from the drawback that they are expensive and, in addition, reduce the possibilities of recycling and reuse of the material, unless special measures are adopted in conjunction with the recovery process to remove the barrier material before the plastic material is reused.
Solutions are also known in the art in which plastic materials of different types are mixed and thereafter molded to form containers by substantially conventional methods. Thus, for example, it is previously known to produce containers of plastic material in which the plastic material consists of a mixture of PET and polyamide. By way of example the polyamide is included in a proportion of between 4 and 10% by weight, preferably at a maximum of 7% by weight. In the production of such containers the two materials are thoroughly intermixed, the thus mixed material is fed to an injection molding machine where the mixture is melted, and the molten mixture is injected to form a preform which is rapidly cooled for the formation of amorphous material, whereupon the preform, after heating, is expanded to form a container.
In the technique described in the preceding paragraph, a certain reduction of the so-called permeability coefficient for oxygen will be achieved. The permeability coefficient is employed as a measure of the permeability of the material in respect of gases. For example, for containers of pure PET of a storage volume of 33 cl, a permeability coefficient for oxygen has been registered of the order of magnitude of between 3 and 4 when the containers are manufactured employing generally applied technology. In the application of the abovedescribed technology employing a mixture of PET and polyamide in the range of proportions stated above, a slightly lower permeability coefficient is obtained which, nevertheless, is relatively high and is of the order of magnitude of between 1 and 3, depending upon the amount of admixed polyamide. In real terms, this implies a prolongation of the shelf-life of, for example, beer from approximately 8 weeks to approximately 16 weeks. Even though a prolongation of the shelf-life to 16 weeks may be of considerable importance, it is, nevertheless, of a marginal nature in many fields of application, in particular in applications within the food industry. The above-described technique of molding containers of PET with an admixture of a minor amount of polyamide has been tested repeatedly. By way of example, it might be mentioned that in five mutually independent trial series, the following results were obtained.
______________________________________ Trial No. Weight percent polyamide Permeability Coefficient ______________________________________ 1 0 3.0 2 2 2.4 3 4 1.8 4 6 1.3 5 7 1.0 ______________________________________
It will be apparent from these results that, for pure PET, the permeability coefficient was measured at 3.0, while, with an admixture of polyamide, the permeability coefficient lay in the range of between 2.4 and 1.0. These disclosed values constitute mean values for 5 different containers or cans for each admixture percentage disclosed in the Table (admixture percentage 0 included, i.e. PET with no admixture of polyamide). For pure PET, the single highest value for the permeability coefficient was 3.4. At an admixture of 2% by weight the change in the permeability coefficient in relation to pure PET is essentially negligible.
The technique for the manufacture of containers of PET and polyamide is conventional and corresponds to the recommendation issued by manufactures of raw material and adapted to suit the properties which these two material types possess.